The present invention relates to a general method for measuring a terminal voltage across a connection terminal of an electric motor.
In the prior art terminal voltages, for example, of an electric motor are detected in order to be able to determine the rotation speed or else the exact position of the electric motor from said terminal voltages. By way of example, it is known in the case of DC motors or electrically commutated motors (EC motors) to detect the position and/or the rotation speed of the drive shaft of the electric motor without the use of a position sensor, which is fitted for example directly to the drive shaft, in a cost-effective manner. To this end, a voltage detection system is provided, said voltage detection system detecting one or more terminal voltages and transmitting said one or more terminal voltages to a control device in order to drive the electric motor. The voltage detection system can be designed as an integral constituent part of the control device of the electric motor or can be designed separately.
In order to be able to operate the electric motors at different rotation speeds, electrical or electronic switching elements, for example relays or transistors, are usually provided, it being possible for the respective connection terminal to be connected to at least one supply voltage or to earth by means of said electrical or electronic switching elements. Accordingly, a desired voltage can be applied and, furthermore, a desired operating mode can be set by deliberately switching over the switching elements on each winding of the electric motor for a desired time. Various types of circuit (H-bridge, 2H, 3H, B6 circuit) are customary for operating electric motors but these do not, in principle, make a distinction in terms of connecting and disconnecting a voltage to/from the connection terminal. However, on account of the inductance of the motor windings and possible voltage drops between the supply voltage or earth and the connection terminal, for example power losses, the voltage across the connection terminal does not correspond exactly to the applied voltage, and therefore explicit detection of the terminal voltage is required in order to be able to determine the position and the rotation speed of the drive shaft from said voltage.
In order to detect the terminal voltage, the voltage detection system comprises a measurement channel which is connected, at the input end, directly to the respective connection terminal in order to tap off the applied voltage and to output said applied voltage at its output. This can be done both by a purely analog connection and by the use of an analog/digital converter which outputs the detected terminal voltage as a digital signal. However, the tapped off terminal voltage is corrupted by the measurement channel, as a result of which it is made more difficult to exactly determine the position and the rotation speed of the drive shaft of the electric motor.
It is known in the art to detect the static response of the measurement channel in order to determine a correction parameter for the static response of the measurement channel, for example in relation to gain and/or a systematic measurement deviation. This correction parameter can be used in order to compensate the static response of the measurement channel. It is also known in the art to carry out automatic compensation of the static response of the measurement channel.
However, the measurement channel also comprises non-linear elements, as a result of which exact detection of the terminal voltage is made more difficult. This relates initially to parasitic capacitances or inductances of linear components, but capacitors and/or coils can also be used explicitly in the measurement channel. In order to keep these non-linear deviations low, high-quality components are used, as a result of which the detection of the terminal voltage is associated with high costs. Active components, for example operational amplifiers, are also sometimes used in order to be able to detect the terminal voltage with the desired degree of accuracy, this being associated with a great deal of effort and likewise high costs.
Proceeding from said prior art, the object of the present invention is to specify a method with which a correction parameter for compensating the non-linear properties of the measurement channel can be determined. A further object of the present invention is to specify a method for detecting the terminal voltage of a motor, which method detects the terminal voltage in a cost-effective manner and with a high degree of accuracy.